1. Field of the Invention
This invention relates to an image forming method using an ion current modulating element such as a photo-sensitive body in the shape of a screen (hereinafter referred to simply as "screen") having a multitude of tiny openings. More particularly, the present invention is concerned with a method for controlling the image conditions on the photo-sensitive body and other like recording media, which is also effective in the image forming method, in which ion current is modulated over a plurality of times by one and the same primary electrostatic latent image.
2. Description of the Prior Art
The most representative electrophotographic techniques known heretofore are classified into the direct method such as the electro-fax method, and the indirect method such as xerographic method. In the image forming method using the abovementioned direct method, the recording member to be used is one that has been subjected to a special treatment, for example, coating of a photoconductive substance such as zinc oxide, etc. on the recording member. On account of this, the image formed on the finished recording member is lacking in its brightness, and the image contrast on its surface has been problematical. Further, owing to the abovementioned special treatment, the recording member has such a disadvantage that, in comparison with common paper, it has a somewhat different sense of touch, and appears to be thick and heavy. Since, in the image forming method due to the latter indirect method, such common paper is used for the recording member to form thereon the desired image, there accrues such advantage that a satisfactory image of high image contrast can be obtained. However, this indirect method has such a disadvantage that, when a toner image is transferred to the recording member, the recording member contacts the surface of the photo-sensitive body, and, moreover, when the residual toner is cleaned, the cleaning device such as a brush, a resilient blade, etc. vigorously contacts the photosensitive body to cause damage to the surface of the photosensitive body at every cleaning operation. As a consequence, the life of the expensive photosensitive body becomes curtailed, which is the cause for increased cost in the image formation.
For removing various disadvantages in the heretofore known electrophotographic techniques, there is such one as disclosed in, for example, Japanese Laid-Open Patent Application No. 48-59840 (corresponding to U.S. Pat. No. 3,713,734). The electrophotographic method as disclosed in this prior art is to utilize a screen in the form of a net or lattice having a multitude of tiny openings in mesh size. The outline of this prior technique is such that the screen is used, through which the ion current is modulated to form an electrostatic latent image on the recording member, after which the recording member with the electrostatic latent image having been formed thereon is developed. That is, according to this electrophotographic method, there is no necessity for developing and cleaning the screen which corresponds to the conventional photosensitive body. Consequently, according to this published electrophotographic method, the screen becomes advantageously usable over an extended period of time. In particular, the method as disclosed in the Japanese Laid-Open Patent Application No. 51-341 (corresponding to U.S. patent application Ser. No. 771,309 and DOLS 2,429,303) has successfully attained improvement in durability of the screen as well as realized image formation by repeatedly utilizing the primary electrostatic latent image, which was once formed, over much more numbers of times than those in the conventional technique (this repeated reproduction operation will hereinafter be called "retention copying"). Although no detailed explanation of that published invention will be given herein, its outline is as follows. First of all, the embodimental construction of this screen is such that a photoconductive member is coated on the electrically conductive base member in a manner to cover the base member with one surface side thereof being exposed to outside, and then an insulating member is provided over the surface of the thus coated electrically conductive member. The image formation is conducted in such a manner that the primary electrostatic latent image formed on the abovementioned screen is utilized to modulate the ion current which has been applied to a chargeable member and to thereby obtain the secondary electrostatic latent image on this chargeable member. For the chargeable member, there may be used electrostatic recording paper and a drum member having thereon an insulating layer as the recording medium (the insulated drum). In case of using the electrostatic recording paper, it is directly developed and fixed by known means, which is used as the reproduced copy. Contrary to this, in the case of using the insulated drum, the secondary electrostatic latent image on the drum is once developed, after which the developed image is transferred to the recording member such as separately provided common paper which will be fixed to serve as the reproduced copy. Therefore, the insulating drum becomes able to be used repeatedly by removing the toner remaining thereon after the image transfer.
By the way, in an image forming device which effects the retention copying by the use of the screen, such as, for example, a reproduction device, there takes place such a situation that the conditions for forming the primary electrostatic latent image becomes deviated from a proper range, which has been established beforehand, due to various changing factors such as difference in the background brightness of an original to be reproduced, changes in environmental conditions surrounding the reproduction device, and others. When the image formation is effected actually under such conditions, the overall image becomes fogged to lower the image contrast, or, conversely, the dark portion in the image (particularly thin lines) is extinguished to result in an ambiguous image. In such a situation, the appropriate image can be obtained by re-adjusting various reproduction conditions such as the amount of image irradiation, amount of electric charge on the screen, and so forth on the basis of detailed inspection of the produced image through the eyes, and then by re-forming the primary electrostatic latent image. This way of obtaining the appropriate image is not preferable, because not only considerable time is wasted for adjustment to set complicated conditions for such adequate image reproduction, but also durability of the screen becomes lowered due to increased number of times of the forming operation of the primary electrostatic latent image onto the screen. Moreover, variations in the forming conditions of the primary electrostatic latent image renders astable the basic conditions for the primary electrostatic latent image formation.